Now it suddenly fails. This is the error message I see if I try to upgrade from command line

$ sudo apt-get install opera-stable
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
opera-stable : Depends: libfontconfig1 (>= 2.9.0) but 2.8.0-3ubuntu9.1 is to be installed
Depends: libglib2.0-0 (>= 2.37.3) but 2.32.4-0ubuntu1 is to be installed
Depends: libharfbuzz0b (>= 0.9.11) but it is not installable
Depends: libnss3 (>= 2:3.15) but 3.17.4-0ubuntu0.12.04.1 is to be installed
Depends: libpango-1.0-0 (>= 1.14.0) but it is not installable
Depends: libpangocairo-1.0-0 (>= 1.14.0) but it is not installable
Recommends: pepperflashplugin-nonfree but it is not installable
E: Unable to correct problems, you have held broken packages.

It seems that there are new dependencies and a mix of older and newer required versions.

Notice that I have installed these versions of those non installable libs (output of dpkg -l)

Hi @pmontrasio! Yes, this is due to a change in the build process, where the build environment was updated to Ubuntu 14.04.

It'll be reverted to the previous dependencies as soon as the next Stable version of Opera is released, and this should be very soon. Sorry for the inconvenience :(.

If you want a newer version of Opera to use in the mean time, you could try using one of the Beta or Developer bleeding-edge versions from http://blogs.opera.com/desktop/. These will run with a separate profile, so they don't affect your main installation of Opera.